The invention relates to a method of stabilising the working point of a predistorter diode of a predistorter for an optical modulator as protection from temperature fluctuations, whereby the predistorter diode is supplied with temperature-dependent voltage in order to adjust the working point.
In order to adjust the working point of a diode in a network which influences the signal, voltage impression is known on the one hand, and current impression on the other. U.S. Pat. No. 5,161,044, FIG. 15, shoes a diode network to linearise the output signal of an external optical modulator through which the light leaving a light source is modulated. The diodes here are connected with voltage sources. As these are semiconductor diodes, their current/voltage characteristic is dependent on temperature. Accordingly, the working point of the diode changes under temperature changes without any special measures, because the voltage supplied to the diode is constant. Also known is guidance of the voltage of predistorter diodes by means of a computer according to a temperature table, whereby a temperature sensor and an analog/digital converter and a digital/analog converter are required. However, this creates a relatively large workload.
In addition, an arrangement with current impression is known from U.S. Pat. No. 5,210,633, FIG. 2. In this arrangement, the DC voltage at the diode changes depending on the level fluctuations of the signal passed through the diode, which can be a high-frequency signal. In this known arrangement, when the diode network consists of a predistorter for linearising the output signal of an optical modulator, it is possible for the working point to be optimally adjusted just for one single value of the amplitude of the high frequency signal. In this known arrangement, it is necessary to guide the current source according to the amplitude for optimum linearisation of the characteristic curve.